1. Field of the Invention
The present invention relates to a glass composition.
2. Description of the Related Art
A glass composition is generally used in various applications as glass products by performing the following procedure: heating various salts, oxides, or the like such as inorganic minerals as raw materials to very high temperatures for forming molten glass, deacrating gas generated through a reaction for fining the molten glass, subsequently homogenizing the molten glass through an operation such as stirring, and then molding the molten glass into a required shape through a specific molding method. An initial problem in the manufacturing of such glass composition involves how to complete the discharging of minute air bubbles which exist in the molten glass. The above can be carried out during melting to provide a homogeneous glass. In other words, reliable fining can be conducted.
Accordingly, various methods have been studied thus far for overcoming the problem of fining. A method, which is most commonly used, involves the adjusting and mixing in advance of a trace additive, a so-called fining, into the raw materials to be melted, and the fine air bubbles are deaerated in the molten glass via a desired chemical reaction at high temperatures. Further, as by another method which may be adopted, a method of maintaining the molten glass in a reduced-pressure or vacuum environment by adjusting an external pressure of the environment itself in which the molten glass resides may be used.
Various additives have been used as fining for the former method, and appropriate fining has been selected in response to the diversification of providing different glass materials for use in the expanded scope of applications of the glass products. Further, various studies have been conducted on the latter method as well for developing many inventions regarding this technique.
Regarding the former method, as proposed in JP 06-293523 A, it is a method of directly introducing a fining agent into an air bubble layer of a glass melting furnace instead of mixing a fining agent into the raw materials in advance. Further, as proposed in JP 11-035338 A, it is an antimony fining agent which replaces an existing fining agent such as an arsenic (As) fining agent that is used thus far.
On the other hand, regarding the latter method, as proposed in JP 2000-128549 A, it is a manufacturing method for glass, which includes a sub-atmospheric pressure step in which the molten glass produced in a melting step is depressurized under sub-atmosphere. However, this method becomes feasible only with the use of a large-scale pressure-reducing vacuum system, thereby posing a problem of high equipment costs. In view of the above, JP 2000-247647 A discloses a furnace material, used for channels of the pressure-reducing vacuum system formed of a prescribed electric fused refractory instead of a precious metal such as platinum. Further, JP 2001-220149 A discloses a revised structure of a bubble collector which discharges the trapped air dissolved in the glass.
Further, dating back 20 years or more, as proposed in U.S. Pat. No. 3,622,296, it is a method of fining glass using helium gas, in which the borosilicate glass is used as an object of the fining.
Of the methods described above, the aforementioned method using a fining has a problem in that the manufacture of glass may not be necessarily stably sustained even when an optimal fining of a specific grade is selected at the beginning of the manufacture. This is because a grade of the glass manufactured may be at an unsatisfactory level with regard to having bubble holes based upon the unavoidable causes such as variabilities in manufacture conditions. Further, trace components are used as additives, and thus, the fining must be capable of uniformly distributed in the molten glass by preventing segregation or the like during mixing of the raw materials.
Further, the latter method using a pressure-reducing device also requires responding to basic problems arising from the theoretical restrictions of the method, even if the higher equipment cost can be handled to some extent. That is, evaporation (also referred to as vaporization) of glass components from the molten glass is hardly prevented when using the pressure-reducing device. Therefore, this method may have to be applied to limited glass applications which are rarely posing problems in evaporation of glass components during melting. Further, taking into account of an evaporation amount of the glass components during melting, the formulation of the raw material components may have to be designed in advance to provide a desired glass composition. Further, sufficient attention may have to be provided on the incidental facilities as compared to the general glass melting facilities so that the evaporated glass components are not carelessly discharged outside of the glass manufacture facilities. As described above, the adoption of this method to the fining of the molten glass forcibly and simultaneously requires the troubleshooting required by overcoming the issues in the method. Therefore, it is not easy for glass manufacturers to adopt this method because the adoption is relatively unfavorable.
Further, U.S. Pat. No. 3,622,296 discloses only the use of helium for the fining of a certain borosilicate glass, but suggests nothing about the glass materials which can be used for effectively fining with helium. Therefore, no attempt had been made on developing the method disclosed in U.S. Pat. No. 3,622,296 and on applying the method to other glass products of higher industrial utility value such as for oxide glass.
As described above, the conventional methods are unable to satisfy an important object of glass manufacture, which is to manufacture a homogeneous glass product without bubbles. Therefore, an improved solution is desired.